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Langmuir : the ACS journal of surfaces and colloids

Imaging ellipsometry of spin-coated membranes: mapping of multilamellar films, hydrated membranes, and fluid domains.


PMID 23281595

Abstract

Imaging ellipsometry (IE) has been applied to generate laterally resolved thickness maps of spin-coated membranes in both the dry and fully hydrated state. Spin-coating offers a convenient preparation method for stacked supported membranes, and in-depth thickness maps for such films can be measured by IE, thereby going beyond topography measurements of the top surface. We find that dry lipid films of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) have a highly ordered multilamellar structure which allows counting of the number of individual bilayers in a thick film from the progression in a concentration series. The average film thickness is approximately proportional to the coating concentration with a constant of proportionality of 5.2 nm/mM and 6.2 nm/mM for POPC and DSPC (1,2-distearoyl-sn- glycero-3-phosphocholine), respectively. The root-mean-square roughness of the dry films is also approximately proportional to concentration with constants of 3.7 nm/mM (DSPC) and 0.87 nm/mM (POPC). Fully hydrated POPC membranes with several stacked bilayers show decreasing thickness for increasing temperature. An apparent excess in thickness by 1.2 nm for the proximal membrane can possibly be linked to the presence of a structured water film next to the solid support. This is supported by modeling of spectroscopic data. Thickness maps of double supported ternary membranes show resolvable liquid-ordered domains in the second membrane while domains are below the resolution limit in the proximal membrane. A thickness difference of 1.69 and 1.89 nm between the liquid-ordered (lo) and liquid-disordered (ld) phases is found for two different ternary membrane compositions. This is approximately twice the height difference measured by AFM on domains, thus indicating that the relative excess thickness of the lo phase is symmetrically distributed.

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